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CORRECTOR MAGNET POWER SUPPLIES FOR INDUS-2 SN Singh, TN Singh, HK Khatwani, ML Gandhi and AC Thakurta, RRCAT Indore (India) THE POWER CIRCUIT Abstract First phase of Fast Orbit Feedback System has been successfully implemented for the correction of fast disturbances to electron beam in Indus-2 Synchrotron to provide stable photon flux to beam users. Air cored combined function fast corrector magnets driven by bipolar current controlled power supplies have been used in a fast beam-position controlled feedback loop to achieve this goal. In order to track the fast varying references generated by the beam position controller, higher power supply current loop bandwidth is desired. The beam position corrector system demands large signal bandwidth of the power supply to be of the order of 100 Hz. This paper describes the design of the power and control circuit of the fast corrector power supplies and the results obtained. The power supplies are rated for ±15 A, ±150 V and have ±50 ppm stability. The developed twostage power supply has a half-controlled thyristor converter followed by a four quadrant switching converter at the output. The load current is sensed using high stability shunt. The control circuit consists of cascaded current and voltage loops. Feed-forward of load voltage has been provided to get better tracking of reference. These power supplies fulfil the requirement of the desired large signal current loop bandwidth. The observed small signal bandwidth is 500 Hz. The power circuit of the supply is shown in Fig 2. It consists of a single phase 550VA step-down mains transformer, followed by single-phase half-controlled SCR bridge rectifier, LC filter, full-bridge inverter feeding the load via L-C filter. The secondary winding of the 550VA step-down transformer voltage is rectified by single-phase halfcontrolled SCR bridge rectifier and filtered by LC filter giving constant 155V DC to full-bridge inverter. The filter capacitor value has been chosen such that its voltage increases by less than 10 volts even if it absorbs the total energy stored in the magnet. The inverter uses MOSFETs switched at 30 kHz employing unipolar switching scheme as shown in Fig 1. Output of the inverter is filtered by LC filter and fed to the magnets. The unipolar switching scheme provides bipolar output voltage with minimum ripple near zero output voltage. Also, the ripple frequency in the output is twice the switching frequency. Provision has also been made to mitigate inrush current into the transformer and the filter capacitor when the supply is first powered. For this, a resistor is inserted in series with the transformer primary. This resistor is bypassed as soon as the filter capacitor voltage reaches 140V. INTRODUCTION RRCAT has taken up implementation of Fast Orbit Feedback (FOFB) system for Indus-2 for better stabilization electron beam orbit which is in turn required to provide specified photon flux stability to the users. It is to be implemented in three phases. The first phase towards this activity was to implement local fast orbit feedback system at beam line 8 which is the most sensitive to beam position variations. The system was implemented and achieved the specified performance of less than 3 micron beam disturbance in both horizontal and vertical directions. Indus-2 FOFB system consists of 2 beam position indicators, 5 combined function air cored fast corrector magnets, 8 power supplies for driving the fast corrector magnets and electronics and software for data acquisition and beam position control. The power supplies for the fast corrector magnets are rated ±150V, ±15A and provide current stability of ±50 ppm or better. They are bipolar current controlled power supplies. The current loop bandwidth of the power supply is 500 Hz. It provides long term current stability of ±50 ppm or better. Figure 1: MOSFET Bridge firing Scheme. The Control Circuit The supply incorporates slow-start feature suitable for current reference of any polarity and any magnitude up to 15A. It has protections against over load current and overtemperature of heat sinks 40uH 8 uH T5 1 2 3 4 8uF 10uFX3/250V + .68uF 5E,25W SHUNT 5408 5408 2500uF/450V 230V/50Hz 120k/2W RTF 60 IXFK140N30P 25mH Figure 2: Power Circuit of FOFB Corrector Magnet Power Supply Figure 3: Schematic Control Circuit for simulating and optimizing the control response 10k/10W TY6004 TY6004 25mH current is sensed using high stability Vishay make shunt. The voltage feedback loop also incorporates filter capacitor current feedback to stabilise the voltage loop. 15mH LOAD The feedback control circuit used for simulating and optimizing the frequency and time response of the feedback control is shown in fig 3. It consists of cascaded load-current feedback loop and load-voltage feedback loop. The control circuit has provision of feed-forwarding the load voltage to get better tracking of reference. The power supply receives current reference in digital form derived from the beam position control loop. The load 40uH Fig 4 The power supplies inside Power supply cabinet. Results Eight power supplies were assembled, tested and deployed for the first phase of fast orbit feedback system. Fig 4 shows the close photograph of the output section of two power supplies. The power supplies performed as per the given specifications and the whole FOFB system achieved the specified performance of less than 3 micron beam disturbance in both horizontal and vertical directions. 24 new power supplies are being developed for the second phase of FOFB planned to be completed by March 2013. References [1] Yi-Da Li and Kuo-Bin Liu, “TPS FAST CORRECTOR MAGNET POWER CONVERTER” , THPO019, Proceedings of IPAC2011, San Sebastián, Spain [2] K.M Ha, J.Y Huang, S.C Kim, J.H Suh, J.H Kim, S.H Jeong, H.S Kang, J. Choi and I.S Ko , “PLS FULLY DIGITAL CONT ROLLED CORRECTOR POWER SUPPLIES” , 10th ICALEPCS Int. Conf. on Accelerator & Large Expt. Physics Control Systems. Geneva. [3] C. Rodrigues, A. R. Silva and L. H. Oliveira, “A NEW FAMILY OF POWER SUPPLIES FOR THE LNLS ORBIT CORRECTORS” , MOPAN003 Proceedings of PAC07, Albuquerque, New Mexico, USA.